The present invention relates in general to microwave moisture indicator and control apparatus, and more particularly to an improved microwave moisture indicator and control device of the transmission type having a pair of microwave transducers for indicating moisture levels of moving webs or material transported thereon, conveyorized items, flowing liquid in conduit systems, or stationary bulk materials such as powdered or granular samples, and producing an output indication of the moisture content of the material or items in the microwave transmission path.
Heretofore, various microwave moisture measuring devices have been disclosed, wherein microwave energy is directed through sheet material and measurements made of attenuation of the microwave energy transmitted through the web to indicate moisture content. Examples of these are found in U.S. Pat. Nos. 3,534,260, 3,639,834 and 3,913,012. Since microwave energy absorbtivity varies only slightly with variations in web moisture content, large differences in moisture produced only slight differences in absorption, so that accurate measurement is difficult to attain with such devices. Efforts to overcome problems of accuracy in such devices have led to the reflective microwave energy moisture measuring devices shown in British Pat. No. 1,052,250 and our earlier U.S. Pat. No. 4,156,843.
An object of the present invention is the provision of an improved novel microwave moisture indicator and control apparatus having a pair of spaced apart microwave transducers using parabolic reflectors to permit moisture in a large area of a web, conveyor path, or body of material to be sensed, whereby variations in moisture across the area being monitored are largely canceled out and more nearly average moisture values are measureable than would occur by conventional reflectance or transmission microwave moisture measuring system.
The importance of being able to make reliable moisture measurements on moving webs or sheets of material moving along a conveyance path, such as warp or other fabric webs, in numerous textile industry applications, has been described in our above identified earlier U.S. Pat. No. 4,156,843, disclosing a reflectance type microwave moisture indicator and control device, and the transmission type microwave moisture indicator apparatus of the present invention is equally suitable for those applications. Additionally, the present invention is designed for a broad range of microwave moisture measurements on any other hygroscopic web material, or to make microwave measurements in the food processing field for measuring the moisture content of food products on conveyors, such as moving conveyors of a non-interferring material like rubber, or moving through chutes or collected in hoppers, and may also be used to detect contaminating moisture in anhydrous liquid flowing through plastic pipe or similar conveyance systems. Moisture content of web-like materials such as wooden boards, veneers, and other webs of variable density may also be advantageously measured and indicated by the large area transmission type microwave moisture indicator and control apparatus of the present invention.